Polyacetal polymers, which are commonly referred to as polyoxymethylenes (POMs), have become established as exceptionally useful engineering materials in a variety of applications. POMs for instance, are widely used in constructing molded parts, such as parts for use in the automotive industry and the consumer appliance industry. POMs, for instance, have excellent mechanical property, fatigue resistance, abrasion resistance, chemical resistance, and moldability.
Although polyacetal resins possess many useful properties, problems have been experienced in the past in combining the polyacetal polymers with various pigments or colorants. Polyacetal polymers, for instance, can be unstable when heated. Adding pigments and colorants to the polymer resin may increase the instability of the polymer. Pigments and colorants have also been found to have adverse effects on the properties of the resulting material.
In view of the above, those skilled in the art have attempted to combine polyacetal polymers with pigments in order to obtain molded articles with a desired appearance without adverse effects on the polymer.
For example, U.S. Pat. No. 5,096,951 discloses color stabilized polyoxymethylene molding compositions. In the '951 patent, polyoxymethylene compositions are disclosed that exhibit improved ambient and heat aged color stability by incorporating a metal deactivator into the composition. The metal deactivator can be used to produce a composition that is natural in color or may include various color pigments, such as azo dyes.
In addition to dyes, attempts have also been made in the past to incorporate metallic pigments into polyoxymethylene polymer compositions. For example, in the past, aluminum pigments have been incorporated into polyacetal polymers to produce molded parts having a satin, painted metal look, especially when molded within molds having a surface temperature of 200° F. or less. Increasing the temperature of the mold surface to greater than 200° F. has been known to increase surface gloss. Unfortunately, however, gloss increases without improving the metallic look. Thus, such parts typically achieve a glossy, but somewhat dull metallic color.
In view of the above, a need exists for a formulation and process for incorporating metallic pigments into polymer compositions, such as polyacetal polymers that can improve the overall metallic appearance of the product. In particular, a need exists for incorporating metallic pigments into polyacetal polymers that produce molded parts having an exterior surface that displays high metallic brilliance.